In a typical radio communications network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” in Universal Mobile Telecommunications System (UMTS) or “eNodeB” in Long Term Evolution (LTE). A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be a downlink or a uplink cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some versions of the RAN, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
An UMTS is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS Terrestrial Radio Access Network (UTRAN) is essentially a RAN using Wideband Code Division Multiple Access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for e.g. third generation networks and further generations, and investigate enhanced data rate and radio capacity.
Specifications for the Evolved Packet System (EPS) have been completed within the 3GPP and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the LTE radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base stations are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations, e.g., eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio base stations without reporting to RNCs.
In today's mobile user equipments (UE), multiple radio transceivers are packaged inside the same device. A UE can be equipped with radio transceivers for one or more cellular Radio Access Technologies and/or one or more “external” wireless systems, i.e., non-cellular communication systems. Examples of such external wireless systems are WiFi, Bluetooth, Global Navigation Satellite System (GNSS), sports or medical related short range wireless systems, cordless telephones, etc. Examples of GNSS are Global Positioning System (GPS), Galileo, Common Positioning. Architecture for Several Sensors (COMPASS), Galileo and Additional Navigation Satellite Systems (GANSS) etc.
There are a variety of user equipments and user equipments are referred with different technical and brand names e.g. USB-dongle, target device, mobile terminal, wireless terminal, wireless terminal used for machine type communication, wireless device used for device to device communication etc. FIG. 1 shows the 3GPP frequency bands around 2.4 GHz industrial, scientific and medical (ISM) bands. The transmit power of one transmitter may be much higher than the received power level of another receiver, which due to extreme proximity of these radio transceivers, can cause interference on the victim radio receiver.
Wi-Fi uses frequency band 2400-2495 MHz in the ISM band. This band is divided into 14 channels, where each channel has a bandwidth of 22 MHz, and 5 MHz separation from other channel with an exception of channel number 14 where separation is 12 MHz. The transmitter of LTE band 40 will affect receiver of WiFi and vice-versa. Since band 7 is a Frequency Division Duplexing (FDD) band so there is no impact on LTE receiver from Wi-Fi transmitter but Wi-Fi receiver will be affected by LTE Uplink (UL) transmitter. Bluetooth operates between 2402-2480 MHz, in 79 channels of 1 MHz bandwidth each. Therefore similar to Wi-Fi, there are interference between band 40 and Bluetooth as well as interference from band 7 UL to Bluetooth Receiver (RX).
Furthermore, the reception of GNSS in the ISM band, e.g. Indian Regional Navigation Satellite System that operates 2483.5-2500 MHz, can be affected by band 7 UL transmission.
In summary some examples of interference scenarios are:                LTE Band 40 radio transmitter (TX) causing interference to ISM radio RX        ISM radio TX causing interference to LTE Band 40 radio RX        LTE Band 7 radio TX causing interference to ISM radio RX        LTE Band 7/13/14 radio TX causing interference to GNSS radio RX        
Note that the frequency bands and radio technologies discussed above are just examples of different possible scenarios. In general the interference can be caused by any radio technology and in any neighboring or sub harmonic frequency band.
To avoid interference from LTE transceiver to other technologies, some interference avoidance solutions can be used in the UE or by the network. Interference avoidance solution can either be done autonomously by the UE, or performed by the network based on an indication from the UE.
In the following the two methods are briefly described:
When a UE experiences a level of In Device Coexistence (IDC) interference that cannot be solved by the UE itself, the UE sends an IDC indication via dedicated Radio Resource Control (RRC) signaling to report the problems, so called Network-controlled UE-assisted Interference avoidance. Indications can be sent by the UE whenever it has problem in ISM DL reception, or in LTE DL reception. Part of the IDC indication message is interference direction, which indicates the direction of IDC interference. The triggering of IDC indication is up to UE implementation, i.e. it may rely on existing LTE measurements and/or UE internal coordination.
The information element, InDeviceCoexIndication, defined in LTE RRC specification, TS 36.331, Rel-11, v. 11.1.0 section 5.6.9 and also shown below describes the message sent by the UE to the radio base station when it experiences problems related to IDC.
The InDeviceCoexIndication message is used to inform the E-UTRAN about IDC problems experienced by the UE, any changes in the IDC problems previously reported by the UE, and to provide the E-UTRAN with information in order to resolve them.
Signalling radio bearer: SRB1
RLC-SAP: AM
Logical channel: DCCH
Direction: UE to E-UTRAN
InDeviceCoexIndication message-- ASN1STARTInDeviceCoexIndication-r11 ::=  SEQUENCE { criticalExtensions     CHOICE { c1       CHOICE {  inDeviceCoexIndication-r11InDeviceCoexIndication-r11-IEs,  spare3 NULL, sPare2 NULL, spare1 NULL }, criticalExtensionsFuture  SEQUENCE { } }}InDeviceCoexIndication-r11-IEs ::= SEQUENCE { affectedCarrierFreqList-r11 AffectedCarrierFreqList-r11OPTIONAL, tdm-AssistanceInfo-r11TDM-AssistanceInfo-r11OPTIONAL, lateNonCriticalExtensionOCTET STRING OPTIONAL, nonCrititcalExtensionSEQUENCE { } OPTIONAL}AffectedCarrierFreqList-r11 ::= SEQUENCE (SIZE (1..maxFreqIDC-r11)) OF AffectedCarrierFreq-r11AffectedCarrierFreq-r11 ::=SEQUENCE { carrierFreq-r11   MeasObjectId, interferenceDirection-r11 ENUMERATED {eutra other,both, spare}}TDM-assistanceInfo-r11 ::= CHOICE { drx-AssistanceInfo-r11SEQUENCE { drx-CycleLength-r11 ENUMERATED {n1} OPTIONAL, drx-Offset-r11ENUMERATED {n1} OPTIONAL, drx-ActiveTime-r11ENUMERATED {n1} OPTIONAL -- The above three parameters (i.e. drx-CycleLength-r11, drx-Offset-r11 and -- drx-ActiveTime-r11) are FFS and need to bediscussed }, idc-SubframePattern-r11SEQUENCE { idc SubframePatternList-r11  IDC-SubframePatternList-r11 }, ... }IDC-SubfamePatternList-r11 ::= SEQUENCE (SIZE (1..maxSubframePatternIDC-r11)) OF IDC-SubframePattern-r11IDC-SubframePattern-r11 ::= CHOICE { subframePatternFDD-r11BIT STRING (SIZE (40)), subframePatternTDD-r11CHOICE { subframeConfig0-r11 BIT STRING (SIZE (70)), subframeConfig1-5-r11BIT STRING (SIZE (10)), subframeConfig6-r11BIT STRING (SIZE(60)) }, ...}-- ASN1STOP
When notified of IDC problems through an IDC indication from the UE, the radio base station can choose to apply Frequency Division Multiplexing (FDM) or Time Division Multiplexing (TDM) solutions.
To assist the radio base station in selecting an appropriate solution, all necessary/available assistance information for both FDM and TDM solutions is included in the IDC indication sent to the radio base station. The IDC indication is also used to update the IDC assistance information, including for the cases when the UE no longer suffers from IDC interference.
The two solutions are explained in more details in the following:
The basic concept of an FDM solution is to move the LTE signal away from the ISM band by performing inter-frequency handover within E-UTRAN. The UE informs the network when operating LTE or other radio signals would benefit or no longer benefit from LTE not using certain carriers or frequency resources. By sending a list of E-UTRA carrier frequencies affected by the IDC problem, the UE will indicate which frequencies are unusable due to in-device coexistence.
The basic concept of a TDM solution is to ensure that the transmission time of a radio signal does not coincide with the reception time of another radio signal of an external wireless system, e.g., a Wireless Local Area Network (WLAN) or GNSS. The UE can signal the necessary information, e.g., interferer type, mode, and possibly the appropriate offset in subframes to the radio base station. The UE can also signal a suggested pattern to the radio base station. Based on such information, the final TDM patterns, i.e., scheduling and unscheduled periods, are configured by the radio base station.
The TDM solutions are divided into different types of methods:                Discontinuous Reception (DRX)-based solution: LTE DRX mechanism is to provide TDM patterns to resolve the IDC issues. The TDM pattern is specified by a total length called DRX periodicity and consists of an active period, scheduling period, and an inactive period, unscheduled period, as shown in FIG. 2. The UE provides the radio base station with a desired TDM pattern consisting of periodicity of the TDM pattern and scheduling period, or unscheduled period. It is up to the network node to decide and signal the pattern that is used by the UE.        
All DRX definitions are according to 3GPP TS 36.321 section 3.1 v.11.0.0. The IDC indication message includes information related to DRX cycle length which indicates the desired DRX cycle length that the E-UTRAN is recommended to configure, DRX offset which indicates the desired DRX starting offset that the E-UTRAN is recommended to configure, and DRX active time which indicates the desired active time that the E-UTRAN is recommended to configure.                Hybrid Automatic Repeat Request (HARQ) process reservation based solution: In this TDM solution, a number of LTE HARQ processes or subframes are reserved for LTE operation, and the remaining subframes are used to accommodate ISM/GNSS traffic. FIG. 3 shows as an example the HARQ reservation process for LTE Time Division Duplexing (TDD) configuration 1, 3GPP TR 36.816 v. 11.2.0 FIG. 5.2.1.2.2-1. In this way interference across in-device co-existing systems can be avoided since UE does not transmit in certain subframes during which it receives ISM/GNSS signals.        
Subframe reservation pattern is sent to the UE in the form of a bitmap based on the assistance information reported by the UE. The provided bitmap is a list of one or more subframe patterns indicating which HARQ process E-UTRAN is requested to abstain from using. Value 0 indicates that E-UTRAN is requested to abstain from using the subframe. As an example the bit sequence 1111110100 means that subframes number 7, 9 and 10 must not be used. The size of bit string for FDD is 40, and for TDD is 70, 10, 60 for subframe configurations 0, 1-5, and 6, respectively. The key point here is that the reserved subframes should comply with LTE release 8/9 UL and DL HARQ timing.
The UE can also deny LTE subframes autonomously, to avoid interfering with important signaling in other radio technologies. During the denied subframes the UE does not transmit any signal. It may also not receive any signal. The amount of denials is limited using a maximum allowed denial subframes over a denial validity period. Both the maximum denial subframes and the denial validity period are configured by the radio base station. Configuring a proper denial rate is left up to radio base station implementation, but the UE decides which subframes are denied, without any further feedback to the radio base station. That is why it is also called as, ‘autonomous denials’. If the radio base station does not configure any denial rate, the UE shall not perform any autonomous denials.
The information element ‘IDC-Config’ defined in LTE RRC specification, TS 36.331, v. 11.1.0 section 6.3.6, and also shown below describes the message sent by the E-UTRAN (eNB) to the UE to release or setup autonomous denial parameters, autonomousDenialSubframes and autonomousDenialValidity.
OtherConfig Information Element
-- ASN1STARTOtherConfig-r9 ::= SEQUENCE {reportProximityConfig-r9 ReportProximityConfig-r9OPTIONAL,--Need ON ... , [[ idc-Config-r11IDC-Config-r11OPTIONAL -- Need ON]]}IDC-Config-r11 ::= CHOICE {releaseNULL,SetupSEQUENCE {autonomousDenialParameters-r11SEQUENCE {autonomousDenialSubframes-r11 ENUMERATED {n2, n5, n10, n15,n20, n30 spare2, spare1},autonomousDenialValidity-r11ENUMERATED {sf200, sf500, sf1000, sf2000,spare4, spare3, spare2, spare1}}OPTIONAL, -- Need OR...}}ReportProximityConfig-r9 ::= SEQUENCE {proximityIndicationEUTRA-r9 ENUMERATED{enabled}   OPTIONAL,  -- Need ORproximityIndicationUTRA-r9  ENUMERATED{enabled}   OPTIONAL -- Need OR}-- ASN1STOP
Radio Resource Management (RRM) measurement
Several radio related measurements are used by the UE or the radio network node to establish and keep the connection, as well as ensuring the quality of a radio link.
The RRM measurements are used in RRC idle state operations such as cell selection, cell reselection, e.g. between E-UTRANs, between different Radio Access Technologies (RAT), and to non-3GPP RATs, and minimization of drive test (MDT), and also in RRC connected state operations such as for cell change, e.g. handover between E-UTRANs, handover between different RATs, and handover to non-3GPP RATs.
Cell ID Measurements
The UE has to first detect a cell and therefore cell identification e.g. acquisition of a Physical Cell Identity (PCI), is also a signal measurement. The UE may also have to acquire the Cell Global ID (CGI) of a UE.
In HSPA and LTE the serving cell can request the UE to acquire the System Information (SI) of the target cell. More specifically the SI is read by the UE to acquire the CGI, which uniquely identifies a cell, of the target cell. The UE also be requested to acquire other information such as Closed Subscriber Group (CSG) indicator, CSG proximity detection etc from the target cell.
The UE reads the SI of the target cell, e.g. intra-, inter-frequency or inter-RAT cell, upon receiving an explicit request from the serving network node via RRC signaling e.g. from RNC in HSPA or eNode B in case of LTE. The acquired SI is then reported to the serving cell. The signaling messages are defined in the relevant HSPA and LTE specifications.
In order to acquire the SI which contains the CGI of the target cell, the UE has to read at least part of the SI including master information block (MIB) and the relevant system information block (SIB) as described later. The terms SI reading/decoding/acquisition, CGI/ECGI reading/decoding/acquisition, CSG SI reading/decoding/acquisition are interchangeably used but have the same or similar meaning. In order to read the SI to obtain the CGI of a cell the UE is allowed to create autonomous gaps during DL and also in UL. The autonomous gaps are created for example at instances when the UE has to read MIB and relevant SIBs of the cell, which depends upon the RAT. The MIB and SIBs are repeated with certain periodicity. Each autonomous gap is typically 3-5 ms in LTE and UE needs several of them to acquire the CGI.
Signal Measurements
The Reference signal received power (RSRP) and Reference signal received quality (RSRQ) are the two existing measurements used for at least RRM such as for mobility, which include mobility in RRC connected state as well as in RRC idle state. The RSRP and RSRQ are also used for other purposes such as for enhanced cell ID positioning, minimization of drive test etc.
The RSRP measurement provides cell-specific signal strength metric at a UE. This measurement is used mainly to rank different LTE candidate cells according to their signal strength and is used as an input for handover and cell reselection decisions. Cell specific Reference Signals (CRS) are used for RSRP measurement. These reference symbols are inserted in the first and third last Orthogonal Frequency Division Multiplexing (OFDM) symbol of each slot, and with a frequency spacing of 6 subcarriers. Thus within a resource block of 12 subcarriers and 0.5 ms slot, there are 4 reference symbols.
The RSRQ is a quality measure which is the ratio of the RSRP and carrier Received Signal Strength Indicator (RSSI). The latter part includes interference from all sources e.g. co-channel interference, adjacent carriers, out of band emissions, noise etc.
The UE depending upon its capability may also perform inter-RAT measurements for measuring on other systems e.g. HSPA, GSM/GSM Enhanced Data rate for GSM Evolution (EDGE) Radio Access Network (GERAN), Code Division Multiple Access CDMA2000, 1× Round Trip Time (RTT) and High Rate Packet Data (HRPD) etc. Examples of inter-RAT radio measurements which can be performed by the UE are Common Pilot Channel Received Signal Code Power (CPICH RSCP) and CPICH energy per chip over total received power spectral density (Ec/No) for inter-RAT UTRAN, GERAN carrier RSSI for inter-RAT GSM and even pilot strength measurements for CDMA2000 1×RTT/HRPD.
In RRC connected state the UE can perform intra-frequency measurements without measurement gaps. However as a general rule the UE performs inter-frequency and inter-RAT measurements in measurement gaps unless it is capable of performing them without gaps. To enable inter-frequency and inter-RAT measurements for the UE requiring gaps, the network has to configure the measurement gaps. Two periodic measurement gap patterns both with a measurement gap length of 6 ms are defined for LTE:                Measurement gap pattern #0 with repetition period 40 ms        Measurement gap pattern #1 with repetition period 80 ms        
The measurements performed by the UE are then reported to the network, which may use them for various tasks.
The radio network node, e.g. radio base station, may also perform signal measurements. Examples of radio network node measurements in LTE are propagation delay between UE and itself, UL Signal to Interference plus Noise Ratio (SINR), UL Signal to Noise Ratio (SNR), UL signal strength, Received Interference Power (RIP) etc. The radio base station may also perform positioning measurements which are described in a later section.
Radio Link Monitoring Measurements
The UE also performs measurements on the serving cell (aka primary cell) in order to monitor the serving cell performance. This is called as Radio Link Monitoring (RLM) or RLM related measurements in LTE.
For RLM the UE monitors the downlink link quality based on the cell-specific reference signal in order to detect the downlink radio link quality of the serving cell or Primary Cell (PCell).
In order to detect out of sync and in sync the UE compares the estimated quality with the thresholds Qout and Qin respectively. The threshold Qout and Qin are defined as the level at which the downlink radio link cannot be reliably received and corresponds to 10% and 2% block error rate of a hypothetical Physical Downlink Control Channel (PDCCH) transmissions respectively.
In non-DRX downlink link quality for out of sync and downlink link quality for in sync are estimated over an evaluation periods of 200 ms and 100 ms respectively.
In DRX downlink link quality for out of sync and downlink link quality for in sync are estimated over the same evaluation period, which scale with the DRX cycle e.g. period equal to 20 DRX cycles for DRX cycle greater than 10 ms and up to 40 ms.
In non-DRX the out of sync status and in sync status are assessed by the UE in every radio frame. In DRX the out of sync status and in sync status are assessed by the UE once every DRX.
In addition to filtering on physical layer, i.e. evaluation period, the UE also applies higher layer filtering based on network-configured parameters. This increases the reliability of radio link failure detection and thus avoids unnecessary radio link failure and consequently RRC re-establishment. The higher layer filtering for radio link failure and recovery detection would in general comprise the following network controlled parameters:                Hysteresis counters e.g. N310 and N311 out of sync and in sync counters respectively.        Timers e.g. T310 Radio Link Failure (RLF) timer        
For example the UE starts the timer T310 after N310 consecutive Out of Sync (OOS) detections. The UE stops the timer T310 after N311 consecutive In sync (IS) detections. The transmitter power of the UE is turned off within 40 ms after the expiry of T310 timer. Upon expiry of T310 timer the UE starts T311 timer. Upon T311 expiry the UE initiates RRC re-establishment phase during which it reselects a new strongest cell.
In HSPA, similar concepts called out of sync and in sync detection are carried out by the UE. The higher layer filtering parameters, i.e., hysteresis counters and timers, are also used in HSPA. There is also RLF and eventually RRC re-establishment procedures specified in HSPA.
Sampling of Cell Measurement
The overall serving cell or neighbour cell measurement quantity results comprise non-coherent averaging of 2 or more basic non-coherent averaged samples. The exact sampling depends upon the implementation and is generally not specified. An example of RSRP measurement averaging in E-UTRAN is shown in FIG. 4. The FIG. 4 illustrates that the UE obtains the overall measurement quantity result by collecting four non-coherent averaged samples or snapshots, each of 3 ms length in this example, during the physical layer measurement period, i.e. 200 ms, when no DRX is used or when DRX cycle is not larger than 40 ms. Every coherent averaged sample is 1 ms long. The measurement accuracy of the neighbour cell measurement quantity, e.g. RSRP or RSRQ, is specified over this physical layer measurement period. It should be noted that the sampling rate is UE implementation specific. Therefore in another implementation a UE may use only 3 snap shots over 200 ms interval. Regardless of the sampling rate, it is important that the measured quantity fulfills the performance requirements in terms of the specified measurement accuracy.
In case of RSRQ both RSRP, numerator, and carrier RSSI, denominator, should be sampled at the same time to follow similar fading profile on both components. The sampling also depends upon the length of the DRX cycle. For example for DRX cycle >40 ms, the UE typically takes one sample every DRX cycle over the measurement period.
A similar measurement sampling mechanism is used for other signal measurements by the UE and also by the radio base station for UL measurements.
HARQ in LTE
Hybrid Automatic Repeat Request (HARQ) is a process of acknowledging the transmission in downlink or uplink. If the received data is error-free an acknowledgement is sent to the transmitter declaring a positive acknowledgement (ACK). If on the other hand, error detected in the transmission, a negative acknowledgement (NACK) is sent to the transmitter, which means that the packet must be re-transmitted. In LTE, certain timing is agreed between the transmitter and receiver for retransmissions.
In FDD mode, HARQ processes have 8 ms, 8 subframes, round trip time in both UL and DL. This means that 4 ms after transmission an ACK or NACK feedback is expected from the receiver, and if a retransmission is required 4 ms after the feedback, the packet is retransmitted.
In TDD mode since the DL and UL subframes can be different, in different UL/DL configurations, the HARQ timing is different. As an example in UUDL configuration 1, as the table below shows, the ACK/NACK feedback to a downlink transmission can only be sent on subframes number 2, 3, 7, and 8. Therefore the 8 ms round trip time that was mentioned for FDD, cannot be valid for this case.
TABLE 1TDD Uplink-Downlink configurationsDownlink-to-UplinkUplink-Switch-downlinkpointSubframe numberconfigurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
CSI Feedback
In order to utilize the variations in channel in the channel dependent scheduling, LTE UE must provide the radio base station with the channel state report. The channel state report is based on known reference symbols that are transmitted in the DL. The channel state report comprises one or several of the following information:                Rank indication (RI): RI is a recommendation to eNB, on how many layers in the downlink transmission must be used. The RI is only one value which means that the recommended rank is valid across the whole bandwidth        Precoder matrix indication (PMI): PMI indicates the recommended precoder matrix that must be used in the downlink transmission. The recommended precoder matrix can be frequency-selective.        Channel quality indication (CQI): CQI shows the highest modulation and coding that can be used for DL transmission. CQI can be frequency-selective too, which means that multiple CQI reports can be sent for different parts of the bandwidth.        
LTE network can request both periodic and aperiodic CSI reports. In LTE release 8/9 both periodic and aperiodic reports are based on Cell-specific Reference Signal (CRS), but in LTE release 10, the CSI report can also be based on CSI-RS which is used for transmission mode 9.
Positioning
Several positioning methods for determining the location of the target device, which can be any of the wireless device or UE, mobile relay, Personal Digital Assistant (PDA) etc exist. The position of the target device is determined by using one or more positioning measurements, which can be performed by a suitable measuring node or device. Depending upon the positioning the measuring node can either be the target device itself, a separate radio node, i.e. a standalone node, serving and/or neighboring node of the target device etc. Also depending upon the positioning method the measurements can be performed by one or more types of measuring nodes.
The well-known positioning methods are:                Satellite based methods: In this case the measurements performed by the target device on signals received from the navigational satellites are used for determining target device's location. For example either GNSS or A-GNSS, e.g. A-GPS, Galileo, COMPASS, GANSS etc, measurements are used for determining the UE position        Observed Time Difference Of Arrival (OTDOA): This method uses UE measurement related to time difference of arrival of signals from radio nodes, e.g. UE Reference Signal Time Difference (RSTD) measurement, for determining UE position in LTE or Single Frequency Network (SFN)-SFN type 2 in HSPA.        Uplink Time Difference Of Arrival (UTDOA): It uses measurements done at a measuring node, e.g. Location Measurement Unit (LMU), on signals transmitted by a UE. The LMU measurement is used for determining the UE position.        Enhanced cell ID (E-CID): It uses one or more of measurements for determining the UE position e.g. any combination of UE Rx-Tx time difference, BS Rx-Tx time difference, timing advanced (TA) measured by the radio base station, LTE RSRP/RSRQ, HSPA CPICH measurements, CPICH RSCP/Ec/No, Angle of Arrival (AoA) measured by the radio base station on UE transmitted signals etc for determining UE position. The Time Advance measurement is done using use either UE Rx-Tx time difference or BS Rx-Tx time difference or both.        Hybrid methods: It relies on measurements obtained using more than one positioning method for determining the UE position        
In LTE the positioning node, aka Evolved Serving Mobile Location Centre (E-SMLC) or location server, configures the UE, radio base station or LMU to perform one or more positioning measurements depending upon the positioning method. The positioning measurements are used by the UE or by a measuring node or by the positioning node to determine the UE location. In LTE the positioning node communicates with UE using LTE Positioning Protocol (LPP) protocol and with radio base station using LTE Positioning Protocol annex (LPPa) protocol.
Device-to-Device (D2D) Communication
D2D communication enables direct communication between devices e.g. between pair or group of UEs. The D2D communication can be managed by a radio network node or can be done autonomously by the UEs involved in D2D communication. In the former case the D2D UEs maintain a communication link also with the radio network node for control, resource assignment etc. The D2D communication can share the spectrum or frequency band used for cellular communication between UE and radio network node or can use a dedicated spectrum or band.
There are several motivations for introducing the possibility for direct D2D communication as opposed to requiring devices to communicate via an infrastructure node, such as a cellular base station or a wireless access point.
The D2D UE performs the radio measurements, e.g. RSRP, RSRQ, UE Rx-Tx time difference etc, like normal UE on signals transmitted to and/or received from the radio network node. In addition the D2D UE also performs the radio measurements on signals transmitted to and/or received from the other D2D UE with which it communicates. These D2D specific measurements are also similar to SINR, SNR, Block Error Ratio (BLER), RSRP, RSRQ, UE Rx-Tx time difference etc.
Measurements performed at a user equipment or a base station may sometimes be inaccurate due to interferences from a different technology used within the device and may degrade the performance of the communications network.